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Algol (β Per / β Persei / Beta Persei) is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, and the first such star to be discovered, and also one of the first variable stars in general to be discovered. Algol's magnitude changes regularly between 2.3 and 3.5 over a period of 2 days, 20 hours and 49 minutes.
As an eclipsing binary, it is actually two stars in close orbit around one another. Because the orbital plane coincidentally matches the Earth's line of sight, the dimmer star (Algol B) passes in front of the brighter star (Algol A) once per orbit, and the amount of light reaching Earth is temporarily decreased. To be more precise, however, Algol happens to be a triple star system: the eclipsing binary pair is separated by only 0.062 AU, while the third star (Algol C) is at an average distance of 2.69 AU from the pair and the mutual orbital period is 681 days (1.86 years). The total mass of the system is about 5.8 solar masses, and the mass ratios of A, B, C are about 4.5 : 1 : 2.
The variability of Algol was first recorded in 1670 by Geminiano Montanari, but it is probable that this property was noticed long before this time; the name Algol means "demon star," (from Arabic الغول al-ghūl, "the ghoul") which was probably given due to its peculiar behavior. In the constellation Perseus, it represents the eye of the Gorgon Medusa. Astrologically, Algol was considered the most unfortunate star in the sky.
Studies of Algol led to the Algol paradox in the theory of stellar evolution: although components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones, it was observed that the more massive component Algol A is still in the main sequence, while the less massive Algol B is a subgiant star at a later evolutionary stage. The paradox can be solved by mass transfer: when the more massive star became a subgiant, it filled its Roche lobe, and most of the mass was transferred to the other star, which is still in the main sequence. In some binaries similar to Algol, a gas flow can actually be seen.
Algol is 92.8 light years from Earth, however about 7.3 million years ago it passed within 9.8 light years. Because the total mass of the system is 5.8 solar masses, and despite the fairly large distance at closest approach, this may have been enough to slightly perturb the solar system's Oort cloud and increase the number of comets entering the inner solar system. However, the actual increase in net cratering rate is believed to have been quite small. 
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